Along with the spread of optical communication technology and development of information processing technology using light, the optical waveguide devices come to be used widely as filter device. In this kind of optical waveguide device realizing various functions by making use of interference of light, generally, since the refractive index and optical path length vary depending on the ambient temperature, it is characterized by fluctuation of the passing band or central wavelength.
Such dependence of central wavelength on temperature in the optical waveguide is generally expressed in formula (1).                                           ⅆ                          λ              0                                            ⅆ            T                          =                                            λ              0                                      n              eq                                ⁢                      (                                          1                L                            ⁢                                                ⅆ                  S                                                  ⅆ                  T                                                      )                                              (        1        )            where λ0 is the central wavelength, neq is the equivalent refractive index of optical waveguide, and 1/L*dsss/dT is the temperature coefficient of optical path length. The temperature coefficient of optical path length can be expressed in formula (2).                                           1            L                    ⁢                                    ⅆ              S                                      ⅆ              T                                      =                                            ⅆ                              n                eq                                                    ⅆ              T                                +                                    n              eq                        ⁢            α                                              (        2        )            where α is the thermal expansion coefficient of the optical waveguide. The thermal expansion coefficient α can be generally approximated by the thermal expansion coefficient of the substrate material. From these formulas (1) and (2), it is known possible to change the temperature dependence of the center wavelength of the optical waveguide device by adjusting the thermal expansion coefficient of the substrate.
That the optical waveguide device has a temperature dependence means that some means is necessary for keeping its characteristic constant regardless of changes of ambient temperature. As such means, for example, it has been attempted to incorporate a Peltier element or heater in the module. But it requires an extra element or part to be incorporated, and not only the cost of the optical waveguide device or the module using it is increased, but also it is contradictory to reduction of size or weight. Accordingly, it has been studied to develop a new technique for realizing temperature independence, that is, rendering the optical waveguide device athermal.
Techniques for lessening the temperature dependence are disclosed, for example, in Japanese Patent Application Laid-open No. 11-174231, Japanese Patent Application Laid-open No. 2000-206348, IEICE Electronics Society Conference 2000, C-3-13, and Electronics Letters 12th October 2000, Vol. 36, No. 21.
FIG. 1 and FIG. 2 show an optical waveguide element substrate, and specifically FIG. 1 shows its plane structure, and FIG. 2 shows a section of the substrate cut in the longitudinal direction in FIG. 1. The optical waveguide element substrate 101 of this example composes a Mach-Zehnder interferometer, which comprises waveguides 103, 104 connected to an input port 1021 and an output port 1022, first and second couplers 105, 106 connected to these waveguides 103, 104, and first and second gratings 107, 108 disposed parallel between these couplers 105, 106.
In this optical waveguide element substrate 101, as shown in FIG. 2, a material of a large thermal expansion coefficient such as aluminum plate 111 is adhered to the reverse side. Due to difference in the thermal expansion coefficient from the aluminum plate 111, a thermal distortion is generated in the optical waveguide element substrate 101. By this thermal distortion, the optical waveguide layer is warped as shown in FIG. 2, and hence the linear expansion coefficient is suppressed to the negative side, so that the temperature dependence of the optical waveguide element substrate 101 is lowered.
By this technique, however, when the light guiding direction and warping direction are deviated, polarization dependence occurs. Accordingly, the optical waveguide element substrate to which this technique can be applied is limited to the optical waveguide element of which light guiding direction is almost one-dimensional or one direction as shown in FIG. 1. In other words, it cannot be applied to the optical waveguide element of which light guiding direction is two-dimensional or X and Y directions. This is explained in the following.
FIG. 3 shows an optical waveguide element substrate of which light guiding direction is two-dimensional, presenting an example of AWG (arrayed waveguide grating). The optical waveguide element substrate 121 of this example comprises a first waveguide 124 or a second waveguide 125 connected to a first port 122 or a second port 123, a first or second slab waveguide 126 or 127 having one end connected to the first optical waveguide 124 or second optical waveguide 125, and a plurality of channel waveguide arrays 128 mutually connecting the other ends of the slab waveguides 126, 127. The optical waveguide element for composing this optical waveguide element substrate 121 has a light multiplexing and demultiplexing function, and has a central wavelength of minimum loss as optical filter characteristic.
FIG. 4 shows a bimetal structure formed by adhering plates of different coefficients of thermal expansion to the optical waveguide element substrate 121 shown in FIG. 3. The optical waveguide element substrate 121 is composed of a silicon substrate 121A, and a waveguide layer 121B formed on its surface, and a plate material 129 of a different thermal expansion coefficient is adhered to the lower side of the silicon substrate 121A in order to compose a bimetal structure.
FIG. 5 compares the central wavelength characteristics as the optical filter characteristics in relation to the substrate temperature in the optical waveguide element substrate shown in FIG. 3, having a plate material for composing a bimetal structure adhered and not adhered to the substrate. This example is disclosed, for instance, in Electronics Letters 12th October 2000, Vol. 36, No. 21. In the diagram, two characteristic lines 131, 132 refer to a case of the substrate to which a plate material is not adhered to compose a bimetal structure, and specifically the characteristic line 131 consisting of black squares showing measuring points shows a case of TM polarization, and the characteristic line 132 of blank squares indicates a case of TE polarization. Other two characteristic lines 133, 134 refer to the bimetal structures, and the characteristic line 133 of black circles shows a case of TM polarization and the characteristic line 134 of white circles indicates a case of TE polarization.
As known from FIG. 5, the bimetal structure is smaller in dislocation of the central wavelength with respect to temperature, and shows better results than the non-bimetal structure. In these two characteristic lines 133, 134, however, the difference of dislocation of central wavelength when the temperature is low and the difference of dislocation of central wavelength when the temperature is high are not equal to each other, and a fluctuation occurs in these differences due to temperature. Therefore, not only the polarization dependence is increased, but also the temperature dependence is increased.
FIG. 6 and FIG. 7 show measures for solving such problem. In the proposal presented at Signal Society General Meeting 2000, C-3-13, an aluminum plate 142 is adhered to a waveguide substrate 141 shown in FIG. 6, and it is processed as shown in FIG. 7. Namely, a region 141A of a specified width including a waveguide 143 is formed in a convex shape thicker than the other region 141B, and the aluminum plate 142 is removed at the position corresponding to the waveguide 143.
Thus, in this proposal, by forming the structure as shown in FIG. 7, it is attempted to cancel the distortion occurring vertically in the waveguide direction. Although such technique is effective in the waveguide device having the guiding direction limited in one direction, it is difficult to apply in the optical waveguide element of which guiding direction is two-dimensional such as the optical waveguide element substrate 121 shown in FIG. 3.
FIG. 8 shows a proposal of adhesion of a substrate having a negative linear expansion coefficient to the upper surface of the optical waveguide element. As disclosed in Japanese Laid-open Patent No. 10-39150, Japanese Patent Application Laid-open No. 2000-292632, Japanese Patent Application Laid-open No. 11-109155, and Japanese Patent Application Laid-open No. 2000-352633, as shown in FIG. 8, a substrate 151 having a negative linear expansion coefficient is adhered to an AWG 152, and the entire linear expansion coefficient is suppressed to the negative side. The arrayed waveguide grating in the diagram is composed same as shown in FIG. 3, that is, multiplex lights of wavelengths λ1, λ2, . . . , λN are entered from the first port 122 side, and optical signals separated into wavelengths λ1, λ2, . . . , λN are taken out from the second port 123 side.
On the other hand, FIG. 9 shows other example of adhesion of a substrate having a negative linear expansion coefficient to the optical waveguide element. In this example, at both sides of an optical waveguide layer 162 having an optical waveguide pattern 161 and an optical waveguide substrate 163 disposed at its lower side, materials of negative linear expansion coefficient composed of substrates 164, 165 having mutually different coefficients of linear expansion are adhered. Such technique is disclosed, for example, in Japanese Patent Application Laid-open No. 11-1099155, and same effects as in the proposal in FIG. 8 are obtained.
Japanese Patent Application Laid-open No. 2000-292632 also proposes a Mach-Zehnder interferometer same as shown in FIG. 9. In the proposal disclosed in Japanese Patent Application Laid-open No. 2000-292632, however, a first carbon sheet having a negative linear expansion coefficient in a fibrous texture direction without covering the 3 dB coupler is adhered to the waveguide surface for composing the optical waveguide layer 162, and a second sheet of a similar structure is adhered to the optical waveguide substrate 163 at the reverse side. Using the first and second carbon sheets (corresponding to the substrates 164, 165 having mutually different coefficients of linear expansion in FIG. 9), by weaving and forming a textile sheet, the ratio of the overall thickness of the substrate having negative coefficients of linear expansion is increased with respect to the thickness of the optical waveguide layer 162, and the suppressing effect of wavelength changes due to temperature is enhanced.
However, in the technologies disclosed in Japanese Patent Application Laid-open No. 10-39150, Japanese Patent Application Laid-open No. 2000-292632, and Japanese Patent Application Laid-open No. 11-109155, from the viewpoint of suppressing expansion or contraction of the waveguide element due to temperature as much as possible, a material having a negative linear expansion coefficient is adhered to both sides of the optical waveguide element. Only by adhering the material having a negative linear expansion coefficient to both sides of the optical waveguide element, the optical waveguide element is warped due to difference in the linear expansion coefficient, in addition to the warp caused by expansion or contraction due to temperature.
Namely, to eliminate the temperature dependence of the optical characteristics of the optical waveguide element, and suppress the thermal deformation of the waveguide element in the longitudinal direction, hitherto, it has been attempted to adhere a correction plate having a smaller linear expansion coefficient than the waveguide element. In such technique, however, bending stress is generated by the difference in the linear expansion coefficient, and thermal deformation in warping direction occurs, and other optical characteristics of the waveguide element deteriorate.
It is hence an object of the invention to present a optical waveguide device capable of suppressing fluctuations of the characteristics of the optical waveguide element possibly occurring due to temperature changes, thereby not requiring temperature adjusting means.